NGC 6946, better known as the Fireworks galaxy, has certainly lived up to its name recently. In a series of observations, an extremely bright flare of X-rays was seen to appear and disappear within a matter of weeks. In a new study, NASA scientists speculate on what could have caused the strange signal.
The puzzle began when NASA’s NuSTAR X-ray telescope was trained on NGC 6946 to observe one of the supernovae that gives the Fireworks galaxy its nickname. At first glance, that supernova was the star attraction, shining brighter than anything else in view. In the image above, it can be seen as a blueish-green spot at the top right.
But when NuSTAR looked at the area again 10 days later, the supernova was being upstaged. Another X-ray signal had bloomed bright, visible in the image as the big green blob towards the bottom left. This appearance within such a short period was unusual in itself, but the mystery deepened when another NASA X-ray observatory, Chandra, observed the galaxy a further 10 days later. The signal had vanished.
"Ten days is a really short amount of time for such a bright object to appear," says Hannah Earnshaw, lead author of the study. "Usually with NuSTAR, we observe more gradual changes over time, and we don't often observe a source multiple times in quick succession. In this instance, we were fortunate to catch a source changing extremely quickly, which is very exciting."
The signal was named ULX-4, for ultraluminous X-ray source. As the name suggests, this is the fourth ULX detected in this galaxy, but its brevity makes it the strangest one. Normally, ULXs last much longer.
So what could it be? While the signal looks quite like a supernova in X-rays, it didn’t give off any light in the visible spectrum, effectively ruling out that explanation. It might have been created by a black hole devouring another object like a star, which can produce bright flares of X-rays. Most of the time, though, these events stretch out much longer than a few weeks. It is possible that a small star was quickly destroyed by a black hole, but this would be quite rare.
Another possibility that the team pitches is that ULX-4 came from a very specific type of neutron star. If one of these small, dense stars happened to be spinning very fast, the magnetic field it produced could act as a barrier that prevents nearby material from falling into the star. This would keep the object dim in X-rays most of the time – except for brief periods when that barrier falters. During these times, material could reach the surface and heat up, producing short-lived X-ray signals that might look like the one detected.
Fortunately, there is a way to test this hypothesis. If the magnetic barrier has wavered once, it should do so again at some point, so astronomers can keep an eye on the spot for any future flare-ups.
The research was published in the Astrophysical Journal.
Source: JPL
